• The solubility of CO 2 /CH 4 in three alkali metal complex ionic liquids was calculated by experiments. • The solubility selectivity of CO 2 /CH 4 was calculated. • The micro mechanism of CO 2 /CH 4 capture and separation was revealed by quantum chemical calculation. Solutions of [Bmim][BF 4 ] ionic liquid (IL) mixed with three alkali metal tetrafluoroborate salts, i.e. LiBF 4 , NaBF 4 and KBF 4 , were used to capture and separate CO 2 /CH 4 at 303.15, 313.15 and 323.15 K under different pressures ranged from 500 to 3000 kPa. CO 2 solubility in three alkali metal doped solutions decreases with the increase of temperature and decreasing pressure. High concentration of alkali metal salts does not show a very improvement for CO 2 absorption, and even reduce CO 2 capture instead in the presence of high concentration of LiBF 4 in ionic liquid. Unexpectedly, KBF 4 has the least solubility in [Bmim][BF 4 ] and LiBF 4 dissolved most in [Bmim][BF 4 ] ionic liquid. On the contrary, the solubility of metal complex ionic liquids to CH 4 decreased after adding alkali metal salts of any concentration. 0.01 mol·L−1 KBF 4 -IL showed the best CO 2 /CH 4 solubility selectivity at 303.15 K, up to 54.5. On a basis of Quantum chemical calculations, binding energies between metal-doped ionic liquids and CO 2 /CH 4 were calculated, following an order of KBF 4 -IL-CO 2 > NaBF 4 -IL-CO 2 > LiBF 4 -IL-CO 2 and LiBF 4 -IL-CH 4 > NaBF 4 -IL-CH 4 > KBF 4 -IL-CH 4 , which is consistent with the experimental results. [BF 4 ]− anions are approaching the metal ions and the larger metal cations, i.e. Na+ and K+, have longer distances with the anions than Li+, which explains the reason of reduced CO 2 solubility when Li+ becomes more. Generally, high concentration of metal ions in ionic liquids are inferior for CO 2 absorption because of the association of MBF 4 with the ionic liquid. Therefore, the results of this paper can lay a theoretical foundation for CO 2 /CH 4 capture separation. [ABSTRACT FROM AUTHOR]